The degree of oxygen saturation of arterial blood is often a vital index of the condition of a patient. Whereas apparatus is available for making accurate measurements on a sample of blood in a cuvette, it is not always possible or desirable to withdraw blood from a patient, and it is obviously impracticable to do so when continuous monitoring is required. Therefore, much effort has been expended in devising an instrument for making the measurement by non-invasive means.
One approach has been to substitute a portion of the body such as a lobe of the ear for the cuvette and measure the difference in light absorption between oxyhemoglobin HbO.sub.2 and deoxyhemoglobin Hb. Unfortunately, however, complications are introduced by the presence of a large number of light absorbers other than HbO.sub.2 and Hb, including, for example, skin and hair.
The following considerations are fundamental to the problem. As blood is pulsed through the lungs by heart action a certain percentage of the deoxyhemoglobin, Hb, picks up oxygen so as to become oxyhemoglobin, HbO.sub.2. By medical definition the oxygen saturation SO.sub.2 =HbO.sub.2 /(Hb+HbO.sub.2). It is this fraction which is to be determined. From the lungs the blood passes through the arterial system until it reaches the capillaries at which point a portion of the HbO.sub.2 gives up its oxygen to support the life process in adjacent cells. At the same time the blood absorbs waste matter from the cells and is made to flow steadily back to the heart by the vascular system.
Increasing the blood flow or perfusion in the ear increases the amount of blood and therefore the amount of HbO.sub.2 reaching the capillaries to such degree that the oxygen thus supplied can far exceed the amount of oxygen consumed by the cells, thereby making the oxygen content of the venous blood nearly equal to the oxygen content of the arterial blood. Inasmuch as the light must obviously pass through both arterial as well as venous blood, the measurement of the relative amounts of Hb and HbO.sub.2 yields an oxygen saturation measurement of SO.sub.2 that approaches the value for arterial blood alone. Unfortunately, the condition of some patients, for example a patient in shock, is often such as to prevent the attainment of sufficient perfusion to yield highly accurate results.
Wood, as described in his U.S. Pat. No. 2,706,927, suggested a way of computing the oxygen saturation from measurements of light absorption at two wavelengths taken under two conditions, (1) a "bloodless" condition in which as much blood as possible is squeezed from the earlobe and (2) a condition of normal blood flow. It was hoped that the "bloodless" measurement would be affected only by the absorbers other than blood and that the normal blood flow measurement would be affected by both the other absorbers and the blood so that a comparison of the readings would indicate the absorption by the blood alone. Unfortunately, the accuracy of the measurements is seriously impaired, not only by the fact that squeezing does not eliminate all the blood but also because it changes the optical coupling between the ear and the optical apparatus. Furthermore, because of wide variations between patients in the effect of absorbers, such as the pigment of the skin and its thickness, a separate calibration must be made for each patient and for each measurement.
Many of these problems have been overcome by apparatus suggested by Shaw in his U.S. Pat. No. 3,638,640 in which light absorption measurements are taken at a number of wavelengths of light. However, in this as well as in all other prior art apparatus, good results have depended on increasing the perfusion in the member of the body being measured so that the blood therein is as close to arterial blood as possible. Whereas perfusion can be increased by artificial methods to the point where accurate results are obtained, there are many situations when the patient's condition makes such methods undesirable or even impossible.